In 2008, the researchers, led by Dr. David H. Sachs and Dr. Megan Sykes, transplanted four donors’ bone marrow along with their kidneys. This appeared to mitigate the problem of organ rejection without the need for loads of immunosuppressive drugs that can plague patients with lifelong side effects.

Doctors prepare the patient’s immune system before the surgery by using drugs that target the immune systems’s T cells — which look for foreign invaders like bacteria or tissue from outside donors — then eliminate them. They then transplant both the donor’s kidney and bone marrow. The bone marrow cells then rebuild the body’s immune system as a chimera, which is a hybrid combination of the patient’s cells and the donor’s cells.

This allows the body to accept the donated organ. Although the procedure is still not widely available, the researchers are conducting the procedure again in 15 kidney patients.

In TIME’s 2008 coverage of the procedure, Dr. Sachs told Alice Park: “It retrains the immune system, fooling it into thinking that the donor tissue is now part of the self. There is no question that during the initial phase, the patient has a lot more difficult time. But they trade that difficulty with what is beginning to look like lifelong suppression [of rejection].”